Man has planned and constructed earth embankments and retaining walls since the onset of his need to create and construct. Early builders recognized the value of reinforcing the material behind retaining walls to minimize the pressures on those walls. The Babylonians reinforced the soils behind their retaining walls with reeds; the Romans used reeds and papyrus; and the Chinese used sticks and other simple materials in backfilling portions of the Great Wall.
The progress of science brought new technology and new ways of supporting embankments. Reinforced concrete and structural steel became the principal tools in retaining earth; these methods were expensive. As an alternative to large, costly concrete and steel earth retaining structures, the French developed a system known as Reinforced Earth (Vidal, 1969, U.S. Pat. No. 3,421,346), where flat steel straps were used as reinforcing elements. Those elements were buried in the backfill behind a retaining wall facing to provide additional shear and tensile strength to the soil and were connected to the wall facing. Davis (1984, U.S. Pat. No. 4,449,857), continuing earlier work by CalTrans (Forsyth, 1978), developed Retained Earth, using steel rods fashioned in the shape of a ladder as reinforcing elements. Hilfiker (1982, U.S. Pat. No. 4,324,508) developed an earth reinforcing system using welded wire mats as reinforcing elements. These reinforced embankments earned the generic title of mechanically stabilized embankments (MSE's).
The Tensar Corporation developed concurrently high density plastic webbing, now known generically as geogrid, which was used as reinforcing elements in the internal reinforcement of steep fill slopes. Woven fabric geogrids coated with plastic entered the market shortly thereafter. Modular blocks soon became the facing elements of choice for non-highway projects and geogrid became its companion earth reinforcing element (Forsburg, 1989, U.S. Pat. No. 4,825,619), (Miner, 1990, U.S. Pat. No. 4,936,713), (Egan, et al, 1999, U.S. Pat. No. 5,911,539). Geogrid also was combined with L-shaped welded wire basket facings for use in constructing temporary retaining walls and embankments during construction of highway overpass projects, by-pass projects, grade separations and other structures requiring temporary retaining walls or embankments.
Corrosion of steel reinforcing elements buried in soil has long been a concern. Galvanization of the steel was adopted as a preventive measure, then the requirement that the backfill surrounding the steel reinforcing elements consist of a “special” (neutral pH) backfill was added. Later work by Sala et al. (1992, U.S. Pat. No. 5,169,266) and studies by private consultants have revealed a significant potential for corrosion of galvanized steel reinforcing elements buried in special backfill where (1) high alkali soils are present and/or (2) salting and sanding of roads occur above or adjacent to MSE's.
Steel reinforcing elements are considered “non-extensible;” i.e. the modulus of elasticity of the steel reinforcing element is greater than the modulus of elasticity of the surrounding backfill. Conversely, geogrid is considered an “extensible” reinforcing element. The design methodology differs between the two types of reinforcing elements, which results in a greater amount of geogrid required than steel reinforcing for similar MSE's. Thus, the materials cost differential between steel reinforcing elements and geogrid reinforcing elements can be negated by the need for a significantly greater amount of geogrid.
Temporary MSE's, which generally have a life of one to three years, often are demolished and the materials (wire basket facing, geogrid and filter cloth) are hauled to a landfill. The costs of hauling those materials to a landfill can approach the cost of the materials, and filling the landfills with those materials is not an environmentally sensitive choice.